Ferrites are useful for microwave applications because they interact with the magnetic component of electromagnetic radiation to produce losses and phase shifts which can be varied with an external magnetic field and which are frequency dependent. These properties make ferrites suitable for applications to circulators, modulators, isolators, phase shifters, and absorbers, among other applications.
The class of hexagonal ferrites with uniaxial magnetic symmetry contains ferrites whose natural resonant frequency can be chosen by selecting appropriate physical structures (e.g. M, W, Z, U . . . ) and by making chemical substitutions in the formula (e.g. 1/2 (CoTi), for Fe, or Co for Zn).
These modifications allow the greatest microwave activity (phase shift and attenuation) to be placed at the frequency ranges of interest. Examples of such uses are in radar absorbers and isolators.
A serious disadvantage of many of the substituted ferrites is that they employ Co ions. Co introduces a strong temperature dependence which shifts the resonant frequency by about 30 MHz/.degree.C. near room temperature. In addition, Co has become relatively expensive. Without the use of Co, the known useful substitutions do not fill all of the frequency ranges of interest, subject to other constraints on their properties.
Various ferrites and methods for producing such ferrites have been disclosed in the prior art. For example, in U.S. Pat. No. 4,425,250 (Hibst), hexagonal ferrites and processes for producing such ferrites are disclosed. The hexagonal ferrites can be of the general formula M.sup.+2 (Me.sup.+2 Ti.sup.+4).sub.2 Fe.sub.12-2x O.sub.19. Such hexagonal ferrites are produced by a ceramic process including mixing of the appropriate oxides, and heating at from about 1,100.degree.-1,200.degree. C. Thereafter, the sintered crystal-like conglomerates formed are milled, generally in the presence of water, to give a powder with a particle size about 1 micrometer. However, it should be appreciated that the milling creates crystal defects in the particles, resulting in poor magnetic properties. Other hexagonal ferrites of this type are disclosed in U.S. Pat. No. 4,664,831 (Hibst et al), U.S. Pat. No. 4,543,198 (Kamiyama), and U.S. Pat. No. 4,764,300 (Hibst et al). This latter patent further discloses the pre-sintering at 1,100.degree.-1,300.degree. C. and the partial curing of the crystal defects caused by milling by heating after the milling procedure or by sintering process.
In U.S. Pat. No. 4,561,988 (Nagai et al), a process for the production of a barium ferrite of the general formula BaFe.sub.12-x-y M.sub.x Ti.sub.y O.sub.19 is disclosed. This process includes autoclaving of an aqueous suspension. Another similar hexa-ferrite is disclosed in U.S. Pat. No. 4,781,852 (Kacxur et al).
A method for producing hexagonal ferrites using a homogenous mixture of cations and one or more thermally decomposable anions is disclosed in U.S. Pat. No. 4,786,430 (Mair).